1. Field of the Invention
The present invention relates to a heat-insulating material made of a porous sintered body and having excellent in insulation properties at a high temperature of 1000° C. or more.
2. Description of the Related Art
Since a porous sintered body has a lower bulk density and lower thermal conductivity in comparison with denser ceramics, and it is widely used as heat-insulating materials.
For example, Japanese Patent Application Publication No. 2011-1204 (Patent Document 1) discloses a heat-insulating material obtained by compression molding the raw materials containing ultrafine fumed oxide as a main raw material and ceramic fine powder etc., the heat-insulating material having a particle structure such that a graph of the pore size distribution shows peaks respectively present in a pore diameter range of from 0.01 to 0.1 μm and in a pore diameter range of 10 to 1000 μm but shows no peak within a range of from 0.1 to 10 μm.
Further, Japanese Patent Application Publication No. 2012-229139 (Patent Document 2) discloses porous ceramics formed of MgAl2O4, having a porosity of 40 to 95 volume %, and having at least one pore diameter distribution peak within a range of from 10 to 365 nm.
However, the heat-insulating material described in Patent Document 1 aims at controlling shrinkage when drying after moisture absorption. Although it has heat resistance at a temperature exceeding around 500° C., particle growth of an ultrafine fumed oxide takes place in a high temperature range of 1000° C. or more, thus causing the number of pores to decrease, the porosity to be reduced, insulation properties to be degraded with changes of the pore diameter distribution, and the heat-insulating material to be deformed and shrunk.
On the other hand, the porous ceramic described in Patent Document 2 requires that at least one pore diameter distribution peak is within a range of from 10 to 365 nm, i.e. fine pores are present. However, it is not clear how many fine pores are present or how much the thermal conductivity is. Therefore, it is not sufficiently clarified how much heat insulation capability it has in which temperature range.